Traditionally, the surface of the thermoplastic sheets, such as PMMA sheets, has been protected during their transport and their handling before the thermoforming by sticking sheets of paper on them. The adhesives used for this purpose have been generally water-soluble adhesives applied in solution form. However, it was necessary to apply these adhesives very uniformly to the surface of the sheet.
Since an application of an adhesive is very difficult to implement on an industrial scale, it was then proposed to replace the paper with an ethylene polymer film. This eliminates the need to use the adhesive. In the case of the protection of cast PMMA sheets, the ethylene polymer film is applied with the aid of hot rolls. However, when the PMMA sheet is freed from its ethylene polymer film before the thermoforming, the high static charge present on the sheets introduces the disadvantages that the people handling these sheets may be subjected to an electric shock. Also, dust is attracted more strongly into contact with the sheets.
To overcome these disadvantages, it has been proposed, in accordance with French Patent No. 2,349,425, to keep the polyethylene protection on the PMMA sheet during the thermoforming. This also provides the additional advantage of keeping this same protection on the wrought product. In many cases, and especially in that of the products intended to form sanitaryware, it was essential to apply a new protective coating because of the many handling operations which still remain to be performed before the fixture is installed on the consumer's premises.
This known process relates, therefore, to the thermoforming of a PMMA sheet, defined as having a reduced viscosity of at least 1.5 and, preferably, of at least 3.0, such as determined at 25.degree. C. on a chloroform solution containing 1 g of the polymer in 1000 cm.sup.3 of solution. The PMMA sheet is protected, on at least one of its surfaces, by a film, in intimate contact therewith, of a polyethylene which has been subjected to a corona discharge treatment (or corona treatment) at least on the face adhering to the PMMA sheet. In accordance with this known process, the protected PMMA sheet is heated to a temperature of at least 140.degree. C., until it is softened by the heat. The heating is not sufficiently long for the sheet to be damaged by the changes induced in the polyethylene by the heat treatment to which it is subjected. The assembly, consisting of the sheet and its protection, is then deformed until the desired final shape is obtained. The polyethylene is still in intimate contact with the sheet. In accordance with this French patent, it is recommended to employ a film of polyethylene which has a melt flow index, determined according to ASTM Standard 1238-70, ranging from 1 to 4 g/10 min., the preference being given to the range from 2 to 3 g/10 min.
Apart from the conditions which it must fulfil to be capable of being used in the process indicated above, and which are sometimes the result of a compromise to satisfy at the same time the requirements of protection of the PMMA sheet, of temporary adhesiveness and of thermoforming, the ethylene polymer film must be capable, after the thermoforming, of retaining its integrity of appearance and of color until the time when it is to be pulled off. At that time, it must be capable of being pulled off easily, even after prolonged storage. Once pulled off, it must not have left any traces, nor have induced interfering microdeformations (micropits) on the PMMA sheet.